For example, a reverse osmosis system serves the purpose of making drinking water from salt water or sewage water, in the following combined to the term “feed water”. For this purpose, the feed water is brought to a relatively high pressure, for example 80 bar or more, by the high-pressure pump, and supplied to the inlet of the membrane unit. In the membrane unit is arranged a semi permeable membrane that retains the dirt or the salt of the feed water and only permits the passage of cleaned water, called permeate. This increases the salt and dirt concentration in the remaining feed water. Feed water with the increased concentration, also called concentrate, is discharged from the membrane unit through the concentrate outlet. However, this concentrate is still under a relatively high pressure of, for example, 60 to 70 bar, so that there is a wish to regain the energy content stored in the concentrate. However, the invention is not limited to the use of water as fluid.
It is, therefore, known to connect the concentrate outlet to a pressure exchanger. On the concentrate side, the pressure exchanger is supplied with concentrate under the increased pressure. On the other side, the feed side, feed water is supplied to have its pressure increased by the concentrate. Pressure exchangers are, for example, known from DE 37 81 148 T2, U.S. Pat. No. 5,338,158, U.S. Pat. No. 5,988,993, WO 99/17028 A1, U.S. Pat. No. 6,540,487 B2 or U.S. Pat. No. 7,214,315 B2. The latter publication also mentions the use in a reverse osmosis process.
Usually, however, the pressure exchanger or pressure converter cannot transfer the complete pressure of the concentrate to the feed water. Also the membrane unit has a certain pressure drop between the inlet and the concentrate outlet. In order to bring the share of the feed water that has been brought to a higher pressure by means of the pressure exchanger up to the final, required pressure for the membrane unit, a booster pump is therefore required. This booster pump has to be driven, meaning that it requires additional energy. The control of the booster pump is problematic. When the booster pump supplies too much fluid, it may happen that it does not only supply feed water, but also concentrate, so that the concentrate concentration on the inlet side of the membrane increases, which again results in a reduction of the efficiency. This phenomenon is called “mixing”. With a higher salt concentration, the energy consumption increases. Many systems, therefore, use one or more flow meters to prevent a mixing of the fluids.